Characteristics of transmural potential changes associated with the proton‐peptide co‐transport in toad small intestine.

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Abstract
1. Ionic dependence and kinetic properties of the peptide‐evoked potentials across everted toad intestine were investigated with eighteen dipeptides and four tripeptides. All peptides evoked saturable increases in the mucosal negativity regardless of the presence of Na+. 2. The peptide‐evoked potentials recorded in the absence of Na+ were sensitive to external pH (pHo); lowering pHo from 7.4 to 6.5 and 5.5 caused stepwise increases in their amplitude. 3. Loading epithelial cells with 9‐aminoacridine or acetate caused a significant increase or decrease in amplitude of the Gly‐Gly‐evoked potential, suggesting intracellular alkalinization or acidification also has a great influence on the peptide‐evoked potential. 4. Kinetically, Na+‐independent peptide‐evoked potentials conformed to simple Michaelis‐Menten kinetics, and lowering pHo caused a decrease of the half‐saturation concentration (Kt) for Gly‐Gly without changing the maximum potential difference increase. Similar affinity‐type kinetic effect was also seen for Gly‐Gly influx. 5. Simultaneous measurements of Gly‐Gly‐induced increase in short‐circuit current and Gly‐Gly influx revealed that the coupling ratio of H+ and Gly‐Gly flows was 1.78 +/‐ 0.12, suggesting the stoichiometry of the H+‐peptide co‐transport being 2:1. 6. Kinetic analyses of the peptide‐evoked potentials indicated that all glycyl‐dipeptides tested (Gly‐Gly, Gly‐Pro, Gly‐Sar, Gly‐Leu, Gly‐Phe) and other dipeptides (Ala‐Ala, Ala‐Phe, Phe‐Ala) shared a common carrier. Gly‐Gly‐Gly and Ala‐Ala‐Ala were also found to share the same carrier, while Phe‐Phe, Leu‐Leu and Phe‐Leu appeared to be transported by a different carrier. 7. Kt values for di‐ and tripeptides, which apparently shared a common carrier, fell in a narrow range (0.5‐2.2 mM). There was no clear correlation between 1/Kt value and molecular weight.

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